Adding a watermark signature to a photo instantly makes it ugly and tacky. Let people view your photos without some ugly watermark blocking it. Only painters are allowed to put visible signatures on their works.
Maybe people think the watermark will prevent their photos from being “stolen”? Indeed, an ugly watermark does make a photo less appealing to borrow. But so what if someone borrows your photo? There are tens of billions of photos on Instagram, and more are being posted every day. With so many photos floating around cyberspace, your photo isn’t anything special, so don’t fool yourself into thinking that a relatively low-resolution version of your photo is worth any money.